233 research outputs found
Restriction of measles virus gene expression in acute and subacute encephalitis in Lewis rats
No abstract availabl
Photometric and Spectroscopic Analysis of Cool White Dwarfs with Trigonometric Parallax Measurements
A photometric and spectroscopic analysis of 152 cool white dwarf stars is
presented. The discovery of 7 new DA white dwarfs, 2 new DQ white dwarfs, 1 new
magnetic white dwarf, and 3 weak magnetic white dwarf candidates, is reported,
as well as 19 known or suspected double degenerates. The photometric energy
distributions, the Halpha line profiles, and the trigonometric parallax
measurements are combined and compared to model atmosphere calculations to
determine the effective temperature and the radius of each object, and also to
constrain the atmospheric composition. New evolutionary sequences with C/O
cores with thin and thick hydrogen layers are used to derive masses and ages.
We confirm the existence of a range in Teff between 5000 and 6000K where almost
all white dwarfs have H-rich atmospheres. There is little evidence for mixed
H/He dwarfs, with the exception of 2 He-rich DA stars, and 5 C2H white dwarfs
which possibly have mixed H/He/C atmospheres. The DQ sequence terminates near
6500K, below which they are believed to turn into C2H stars. True DC stars
slightly above this temperature are found to exhibit H-like energy
distributions despite the lack of Halpha absorption. Attempts to interpret the
chemical evolution show the problem to be complex. Convective mixing is
necessary to account for the non-DA to DA ratio as a function of temperature.
The presence of helium in cool DA stars, the existence of the non-DA gap, and
the peculiar DC stars are also explained in terms of convective mixing,
although our understanding of how this mechanism works needs to be revised. The
oldest object in our sample is about 7.9 Gyr or 9.7 Gyr old depending on
whether thin or thick hydrogen layer models are used. The mean mass of our
sample is 0.65 +/- 0.20 Msun.Comment: Accepted by ApJ Suppl (~April 2001); 79 pages incl. 25 figure
The Formation Rate, Mass and Luminosity Functions of DA White Dwarfs from the Palomar Green Survey
Spectrophotometric observations at high signal-to-noise ratio were obtained
of a complete sample of 347 DA white dwarfs from the Palomar Green (PG) Survey.
Fits of observed Balmer lines to synthetic spectra calculated from
pure-hydrogen model atmospheres were used to obtain robust values of Teff, log
g, masses, radii, and cooling ages. The luminosity function of the sample,
weighted by 1/Vmax, was obtained and compared with other determinations. The
mass distribution of the white dwarfs is derived, after important corrections
for the radii of the white dwarfs in this magnitude-limited survey and for the
cooling time scales. The formation rate of DA white dwarfs from the PG is
estimated to be 0.6x10^(-12) pc^(-3) yr^(-1). Comparison with predictions from
a theoretical study of the white dwarf formation rate for single stars
indicates that >80% of the high mass component requires a different origin,
presumably mergers of lower mass double degenerate stars. In order to estimate
the recent formation rate of all white dwarfs in the local Galactic disk,
corrections for incompleteness of the PG, addition of the DB-DO white dwarfs,
and allowance for stars hidden by luminous binary companions had to be applied
to enhance the rate. An overall formation rate of white dwarfs recently in the
local Galactic disk of 1.15+/-0.25x10^(-12) pc^(-3) yr^(-1) is obtained. Two
recent studies of samples of nearby Galactic planetary nebulae lead to
estimates around twice as high. Difficulties in reconciling these
determinations are discussed.Comment: 73 pages, 18 figures, accepted for publication in the ApJ Supplemen
Additional Ultracool White Dwarfs Found in the Sloan Digital Sky Survey
We identify seven new ultracool white dwarfs discovered in the Sloan Digital
Sky Survey (SDSS). The SDSS photometry, spectra, and proper motions are
presented, and additional BVRI data are given for these and other previously
discovered ultracool white dwarfs. The observed colors span a remarkably wide
range, qualitatively similar to colors predicted by models for very cool white
dwarfs. One of the new stars (SDSS J1251+44) exhibits strong collision-induced
absorption (CIA) in its spectra, while the spectra and colors of the other six
are consistent with mild CIA. Another of the new discoveries (SDSS J2239+00A)
is part of a binary system -- its companion is also a cool white dwarf, and
other data indicate that the companion exhibits an infrared flux deficiency,
making this the first binary system composed of two CIA white dwarfs. A third
discovery (SDSS J0310-00) has weak Balmer emission lines. The proper motions of
all seven stars are consistent with membership in the disk or thick disk.Comment: Accepted for Astrophysical Journal. 16 pages (includes 3 figures
The First Substellar Subdwarf? Discovery of a Metal-poor L Dwarf with Halo Kinematics
We present the discovery of the first L-type subdwarf, 2MASS
J05325346+8246465. This object exhibits enhanced collision-induced H
absorption, resulting in blue NIR colors (). In
addition, strong hydride bands in the red optical and NIR, weak TiO absorption,
and an optical/J-band spectral morphology similar to the L7 DENIS 02051159AB
imply a cool, metal-deficient atmosphere. We find that 2MASS 0532+8246 has both
a high proper motion, = 2\farcs60\pm0\farcs15 yr, and a
substantial radial velocity, km s, and its
probable proximity to the Sun (d = 10--30 pc) is consistent with halo
membership. Comparison to subsolar-metallicity evolutionary models strongly
suggests that 2MASS 0532+8246 is substellar, with a mass of 0.077 M
0.085 M_{\sun} for ages 10--15 Gyr and metallicities Z_{\sun}. The discovery of this object clearly indicates that star
formation occurred below the Hydrogen burning mass limit at early times,
consistent with prior results indicating a flat or slightly rising mass
function for the lowest-mass stellar subdwarfs. Furthermore, 2MASS 0532+8246
serves as a prototype for a new spectral class of subdwarfs, additional
examples of which could be found in NIR proper motion surveys.Comment: 9 pages, 3 figures, accepted to Ap
Discovery of New Ultracool White Dwarfs in the Sloan Digital Sky Survey
We report the discovery of five very cool white dwarfs in the Sloan Digital
Sky Survey (SDSS). Four are ultracool, exhibiting strong collision induced
absorption (CIA) from molecular hydrogen and are similar in color to the three
previously known coolest white dwarfs, SDSS J1337+00, LHS 3250 and LHS 1402.
The fifth, an ultracool white dwarf candidate, shows milder CIA flux
suppression and has a color and spectral shape similar to WD 0346+246. All five
new white dwarfs are faint (g > 18.9) and have significant proper motions. One
of the new ultracool white dwarfs, SDSS J0947, appears to be in a binary system
with a slightly warmer (T_{eff} ~ 5000K) white dwarf companion.Comment: 15 pages, 3 figures, submitted to ApJL. Higher resolution versions of
finding charts are available at
http://astro.uchicago.edu/~gates/findingchart
A search for kilogauss magnetic fields in white dwarfs and hot subdwarf stars
We present new results of a survey for weak magnetic fields among DA white
dwarfs with inclusion of some brighter hot subdwarf stars. We have detected
variable circular polarization in the Halpha line of the hot subdwarf star
Feige 34 (SP: sdO). From these data, we estimate that the longitudinal magnetic
field of this star varies from -1.1 +/- 3.2 kG to +9.6 +/- 2.6 kG, with a mean
of about +5 kG and a period longer than 2 h. In this study, we also confirm the
magnetic nature of white dwarf WD1105-048 and present upper limits of kilogauss
longitudinal magnetic fields of 5 brightest DA white dwarfs. Our data support
recent finding that 25% of white dwarfs have kilogauss magnetic fields. This
frequency also confirms results of early estimates obtained using the magnetic
field function of white dwarfs.Comment: accepted for publication in the Astrphysical Journa
Detection of H Alpha Emission in a Methane (T-Type) Brown Dwarf
We report the detection of H{} emission in the T dwarf (methane brown
dwarf) 2MASSW J1237392+652615 over three days using the Keck Low Resolution
Imaging Spectrograph. The measured line flux, log(L/L) =
4.3, is roughly consistent with early M dwarf activity levels and
inconsistent with decreasing activity trends in late M and L dwarfs. Similar
emission is not seen in two other T dwarfs. We speculate on several mechanisms
that may be responsible for emission, including a strong magnetic field,
continuous flaring, acoustic heat generation, and a close ( 4 - 20
R) interacting binary, with the cooler component overflowing its Roche
lobe. We suggest that the M9.5Ve PC 0025+0447 could be a warm analogue to 2MASS
J1237+65, and may be powered by the latter mechanism.Comment: 14 pages, 3 figures, accepted by A
Ligand-Receptor Interactions
The formation and dissociation of specific noncovalent interactions between a
variety of macromolecules play a crucial role in the function of biological
systems. During the last few years, three main lines of research led to a
dramatic improvement of our understanding of these important phenomena. First,
combination of genetic engineering and X ray cristallography made available a
simultaneous knowledg of the precise structure and affinity of series or
related ligand-receptor systems differing by a few well-defined atoms. Second,
improvement of computer power and simulation techniques allowed extended
exploration of the interaction of realistic macromolecules. Third, simultaneous
development of a variety of techniques based on atomic force microscopy,
hydrodynamic flow, biomembrane probes, optical tweezers, magnetic fields or
flexible transducers yielded direct experimental information of the behavior of
single ligand receptor bonds. At the same time, investigation of well defined
cellular models raised the interest of biologists to the kinetic and mechanical
properties of cell membrane receptors. The aim of this review is to give a
description of these advances that benefitted from a largely multidisciplinar
approach
Magnetic Field Generation in Stars
Enormous progress has been made on observing stellar magnetism in stars from
the main sequence through to compact objects. Recent data have thrown into
sharper relief the vexed question of the origin of stellar magnetic fields,
which remains one of the main unanswered questions in astrophysics. In this
chapter we review recent work in this area of research. In particular, we look
at the fossil field hypothesis which links magnetism in compact stars to
magnetism in main sequence and pre-main sequence stars and we consider why its
feasibility has now been questioned particularly in the context of highly
magnetic white dwarfs. We also review the fossil versus dynamo debate in the
context of neutron stars and the roles played by key physical processes such as
buoyancy, helicity, and superfluid turbulence,in the generation and stability
of neutron star fields.
Independent information on the internal magnetic field of neutron stars will
come from future gravitational wave detections. Thus we maybe at the dawn of a
new era of exciting discoveries in compact star magnetism driven by the opening
of a new, non-electromagnetic observational window.
We also review recent advances in the theory and computation of
magnetohydrodynamic turbulence as it applies to stellar magnetism and dynamo
theory. These advances offer insight into the action of stellar dynamos as well
as processes whichcontrol the diffusive magnetic flux transport in stars.Comment: 41 pages, 7 figures. Invited review chapter on on magnetic field
generation in stars to appear in Space Science Reviews, Springe
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